This invention relates to the formation of a copper electrode on ceramic oxide, especially on a sintered ceramic oxide body.
It has long been investigated to bond metal to ceramic materials, particularly oxide ceramic materials. Most commonly known and widely used is the thick film process involving coating and baking of paste. The most widely used pastes are expensive Ag-Pd and Ag pastes which lead to an economical disadvantage although copper pastes are used in limited applications.
The paste is usually applied by the screen printing process which imposes a limit of about 100 .mu.m to the minimum coating line width, which in turn, sets a bar to high integration. Most pastes which contain frits not only have a high interfacial resistance with the ceramic so that they are unsuitable in high frequency circuits, but are also disadvantageous in thermal conductivity. When wet plating such as nickel plating or soldering is carried out as a subsequent process, the plating solution is likely to attack the frits, detracting from electrical conductivity and bond strength as well as reliability. An alternative process is to form copper films by wet plating. This process is successful in increasing electrical conductivity, but results in an insufficient bond strength to use in practice.
It was also developed to bond a copper sheet to a ceramic by forming a Cu-O eutectic liquid phase (see Japanese Patent Application Kokai No. 52-37914). This method, however, is complicated because it involves the oxidation of copper surface and the formation of Cu-O eutectic liquid phase.